1. Field of the Invention
The present invention relates to capacitor element mounting structures in which a plurality of capacitor elements are mounted on a wiring board and capacitor element mounting methods used to mount a plurality of capacitor elements on a wiring board.
2. Description of the Related Art
Recent increases in the capabilities of electronic devices have seen increases in the capacities of laminated ceramic capacitors used as electronic components. A high-permittivity ceramic material such as barium titanate (BaTiO3) is used as a dielectric material in such high-capacity laminated ceramic capacitors.
Such a high-permittivity ceramic material is piezoelectric and electrostrictive, and thus, applying a voltage in a laminated ceramic capacitor that includes a dielectric material configured of a high-permittivity ceramic material produces mechanical distortion.
Accordingly, when an AC voltage or a DC voltage having a superimposed AC component is applied to a high-capacity laminated ceramic capacitor mounted on a wiring board, vibrations are produced due to the mechanical distortion produced in the ceramic material, and the circuit board vibrates as a result of the vibrations being transmitted to the wiring board.
Here, noise referred to as “acoustic noise” is produced in the case where the transmitted vibrations cause the circuit board to vibrate in a frequency range from about 20 Hz to 20 kHz, which corresponds to an audible frequency range.
For example, a DC/DC converter installed in an electronic device converts a DC voltage into a predetermined DC voltage suited for that electronic device and supplies the resulting DC voltage as power, and a laminated ceramic capacitor for decoupling is connected to an input/output circuit of the DC/DC converter in order to reduce noise produced by switching operations. A ripple voltage superimposed on the DC voltage is thus applied to the laminated ceramic capacitor as a result of the switching operations, and the ripple voltage produces mechanical distortion in the laminated ceramic capacitor that has a frequency in the audible frequency range. When this distortion is transmitted to the circuit board, noise is produced in the wiring board.
A variety of techniques have been proposed in order to reduce such noise, such as the technique disclosed in Japanese Unexamined Patent Application Publication No. 2000-232030. The technique disclosed in Japanese Unexamined Patent Application Publication No. 2000-232030 aims to reduce noise by mounting a pair of laminated ceramic capacitors having the same specifications in plane symmetry in corresponding positions on the front and rear surfaces of a wiring board so that vibrations transmitted from one of the laminated ceramic capacitors to the wiring board cancel out vibrations transmitted from the other laminated ceramic capacitor to the wiring board.
Japanese Unexamined Patent Application Publication No. 2002-232110 also discloses a technique for achieving the aforementioned reduction in noise. The technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-232110 aims to reduce noise by mounting a pair of laminated ceramic capacitors near each other on the same main surface of a wiring board so that the long axes thereof are arranged in parallel, where the configuration is such that ripple voltages are applied respectively to the pair of laminated ceramic capacitors to realize an amplitude relationship in which the vibrations in vibration waves transmitted to the wiring board have approximately opposite phases.
However, the technique disclosed in Japanese Unexamined Patent Application Publication No. 2000-232030 can only be applied in the case where the laminated ceramic capacitors can be mounted on the front and rear surfaces of the wiring board, and therefore cannot be applied in the case where the laminated ceramic capacitors can only be mounted on one surface of the wiring board due to the circuitry design of the electronic circuit, the structural design of the electronic device, or the like. There is thus a problem in that there is less freedom with respect to the circuitry design of the electronic circuit and/or the structural design of the electronic device.
There is a further problem in that applying the technique disclosed in Japanese Unexamined Patent Application Publication No. 2000-232030 results in the laminated ceramic capacitors being mounted on the front and rear surfaces of the wiring board, which increases the thickness of the circuit board more than is necessary. This in turn increases the size of the electronic device.
On the other hand, the technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-232110 requires a configuration in which ripple voltages are applied respectively to the pair of laminated ceramic capacitors in order to realize an amplitude relationship in which the vibrations in vibration waves transmitted to the wiring board have approximately opposite phases, resulting in extensive restrictions on the actual mounting layout of electronic components, the circuitry configuration, and so on. Accordingly, there is a problem in that extra space is sometimes necessary on the circuit board in order to realize such a configuration, which can easily lead to an increase in the size of the electronic device.